LB film is an organic film comprising a monomolecular film or multilayer film in which a plurality of monomolecular layers of same or different kinds of organic material are laminated. The thickness of each monolecular film depends the length of the molecules making up the LB film. "LB films" were discovered by Irving Langmuir and Katherine Blodgett and are generally produced in accordance with the following procedure. A material to form the film is dissolved in a suitable solvent. Then a small amount of the solution is allowed to spread over a clean surface of a liquid, in particular, over a surface of pure water, and then the solvent is allowed to evaporate or disperse into the liquid phase. As a result a loosely packed film of the molecules forms on the liquid surface. The surface is then swept by means of a barrier wall to reduce the area of the loosely packed film mechanically, compress the film and increase its density to give a tightly packed monomolecular film. Next, a solid substrate is immersed in the liquid and then raised e.g. vertically through the monomolecular film on the liquid surface while maintaining the surface density of the molecules constituting the monomolecular film. Under these conditions, the monomolecular film transfers to the substrate and is deposited as a monomolecular film on the substrate. By repeated immersing and raising of the substrate, multilayer monomolecular films, that is an LB film, having an integer times thickness of that of the monomolecular film, can be formed on the substrate.
The surface density of the monomolecular film on the liquid surface can be monitored by measuring a surface pressure, that is the difference between the surface tension of the liquid and the surface tension of the area which is covered with the monomolecular film.
In the process of film deposition it is important for almost all LB film forming materials to maintain the surface pressure of the monomolecular film within a narrow range. The reason why regulation is important is that the orientation of the molecules in the monomolecular film changes as the surface pressure changes. That is, when the surface pressure is changed while the LB film is being formed, the orientation in the film is disturbed, and if the orientation is highly disturbed, an LB film might not be produced. The range of tolerance or fluctuation of the surface pressure depends on the material for the LB film and/or the surface pressure for producing the LB film, but it is usual for the surface pressure, to be maintained within 1 mN/m. The surface pressure is usually adjusted by sweeping the barrier wall over the surface to enhance or reduce the area of the liquid surface over which the LB film forming molecules are spread, as described above. Therefore, in order to maintain the surface pressure within a given range it is necessary to control the position of the barrier wall in response to the surface pressure.
However, where because of the rapidity of the deposition or the rigidity of the monomolecular film, the change in position of the barrier wall or the response of the film is too slow compared with changes in the measured surface pressure, feedback control may not operate correctly and the surface pressure may oscillate and consequently the position of the barrier wall may oscillate. For example .+-.3 mN/m of oscillation may occur. Where the amplitude of oscillation is larger than the above value, the instability in surface pressure means that an LB film is not formed. In particular, if the surface pressure attains a high positive value, the monomolecular film on the liquid surface irreversibly collapses, so that such fluctuations must be restricted. Hitherto, the LB films have been produced and film deposition and other procedures have been carried out using film balance apparatus comprising a water tank containing the liquid phase, a barrier wall, a surface pressure gauge and an apparatus for immersing and raising a substrate to be coated or re-coated with an LB film. The quantity of the monomolecular film on the liquid surface and the area of the monomolecular film, depend on the size of the water tank. The area cannot be larger than the surface area of the liquid phase, and actually the monomolecular film is compressed by the barrier wall so that its area is usually smaller than the surface area of the liquid phase. Thus, the total area of the LB film which can be formed on the substrate in a single operation is the same as or less than the area of the monomolecular film which is originally formed on the liquid surface.
Where it is required to produce on a substrate an LB film having a larger area than the area of the monomolecular film formed on the liquid surface, the following procedure is required. First, any residual monomolecular film on the liquid surface is removed to give a clean surface. An example of where this cleaning operation is required is where a monomolecular film remains in insufficient quantity to form a film on the intended substrate in a single operation of immersing and raising the substrate, or where the residual monomolecular film is present in parts which cannot be compressed or controlled by movement of the barrier wall due to apparatus restrictions. Next, the LB film forming material is spread on the liquid surface, and then the molecules of the film forming substance which are present on the liquid surface are compressed to form a monomolecular film which can be used to make the LB films. However, the above described LB film producing process is a batch process so that it is not suitable for mass production.
Various methods have been put forward to meet the need to be able to mass produce LB films. Examples of such disclosures include U.S. Pat. No. 4,783,348 (O. Albrecht et al), an article by O. Albrecht et al, "Industrial Scale Production of LB-Layers", Molecular Electronics, Biosensors and Biocomputers, edited by F. T. Hong, Plenum Press, N.Y. 1989, pages 41-49, and an article by W. Nitsch et al, "Convective Compression in Channel Flow: Behaviour and Transfer of Soluble and Insoluble Films" Thin Solid Films, Vol. 178, 1989, pages 145-155.
Now an apparatus and process proposed by O. Albrecht et al for producing the LB film continuously will be explained with reference to FIG. 2. The apparatus shown in FIG. 2 comprises a channel 20 having three parts, a spreading region (S), a compression region (C) and a depositing region (D). Pure water flows from the region S to the region D through the region C.
First, in the region S, a solution 14 containing a material of which the LB film consists is dropped from a nozzle 19, onto the surface of a flow of pure water. The material spreads out on the water phase 13. In FIG. 2 the molecules of the LB film forming material are represented by the reference numeral 11. Then the spread LB film forming material is transferred in the water flow to the region C while the solvent of the solution 14 evaporates. The LB film-forming material becomes compressed in the region C and a monomolecular film 12 in which the molecules 11 are brought into an orientation is formed on the water surface in the regions C and D. In the region C, a stable flow is necessary to compress the material uniformly. Therefore the channel in the region C has a slope so that the water surface has a slight descent toward the region D.
In the region D, a substrate 15 is repeatedly immersed into the water phase and then raised by transferring means (16) which moves the substrate in the perpendicular direction (17) to the liquid surface. Therefore the monomolecular film 12 is deposited continuously on the substrate 15 to obtain an LB film.
In the process and the apparatus described above, where some material has already been provided to the water phase and some monomolecular film has already been formed in the region C and the region D, fresh material (solution) from the nozzle (19) is transported to the right as viewed in FIG. 2 until the fresh material is stopped by abutment with the end of the already formed monomolecular layer on the liquid surface in the region C. Therefore the new monomolecular layer grows in a direction opposite to the water flow i.e. in an upstream direction. Then, friction which occurs between the new monomolecular film and the water phase gives rise to additional surface pressure contributed by the new monomolecular film so that the surface pressure of the monomolecular film is increased. The surface pressure of the monomolecular film in the region D depends on the form of the container (hereinafter called "water tank") which retains the water phase and the material on the water phase, the water level and the strength of the water flow etc.
As described above, stable water flow is necessary in the region C. To cope with this requirement, the tank is preferably formed so that the water phase in the region C is thin. As a result, the water flow becomes laminar. However, the water phase in the region S must have adequate depth in order to prevent the water flow from being influenced by flow variations in this region. Furthermore, the depth of the water phase in the region D is preferably greater than that in region C in order to permit the substrate 15 to be immersed and to accommodate fluctuation in the amount of water which is drained.
The apparatus shown in FIG. 2 is provided with a region into which water is flowed, a region from which water is drained, a pump for maintaining the water flow, a level sensor in the region D and means for exchanging the water phase and for removing waste film, and these are not shown in FIG. 2.
When the monomolecular film is formed on the water surface by using the apparatus described above, two feedback control loops are preferably operated as the water is caused to flow at an experimentally determined speed. One control loop regulates the water level in the region D by regulating the amount of water in the channel, and another control loop regulates the surface pressure of the monomolecular film on the liquid surface in the region D by regulating the speed of spreading the material on the water surface in the region S. When parameters of these control loops are set suitably, approximately 90% of the surface area in the region C is covered with the monomolecular film. FIG. 3 is a schematic view which illustrates the first and the second control loops.
Referring to FIG. 3, the apparatus has a channel having the spreading region, the compressing region and the depositing region, and water flows from the region S to the region D through the region C continuously. The flow is maintained by a pump 31, and water is provided to the channel through a valve 32 if it is required.
The operation of the first loop is as follows. The water level in the region S is monitored by a level sensor 33 and the water level in the region D is monitored by a level sensor 34. A water level controller 35 regulates the valve 32 which admits fresh supplies of water and a pump 41 which drains water from the region D if the water level is higher than the predetermined level according to outputs from the level sensors 33 and 34 to control the amount of the water in the channel. The levels in regions S and D are therefore regulated by feedback control.
Next, the second control loop will be explained. The surface pressure of the monomolecular film in the region S is monitored by a surface pressure sensor 37 and the surface pressure of the monomolecular film in the region D is monitored by a surface pressure sensor 38.
A surface pressure controller 39 regulates means 40 for dropping the solution from the nozzle 19 to control the speed at which the solution is spread on the region S. Then, feedback control of the surface pressure in the region D is operated.
However, in the technique described above in which two feedback loops are used, the water level and the surface pressure which are regulated by the two loops are not isolated from each other so that there is difficulty in achieving sufficiently precise regulation of the surface pressure of the monomolecular film on the liquid surface in the region D. The reason why the precise regulation is difficult is that the circumstances of the flow change between the upstream end of the channel and the downstream end of the channel. That is, the surface of the water flow at the upstream end is not covered with the monomolecular film, whereas the surface of the water flow in the downstream end is covered with the monomolecular film and the flow at the downstream end is sandwiched between the monomolecular film and the bottom surface of the tank. The effective thickness of the water flow in the region C changes at the border between where the surface is not covered and where the surface is covered with the monomolecular film, and also when the end position of the monomolecular film changes, the total amount of the water in the region C changes. As a result, the water levels in the region S and in the region D change according to the change of the area of the water surface which the monomolecular film covers, i.e. depending on change of the end position of the monomolecular film.
The change of the water level in the region D causes the effective length of the region C, which influences the compression of the material, and the length of the slope in the region C to be changed, which gives rise to a change in the surface pressure of the monomolecular film in the region C. In addition, fluctuation of the water level in the region S causes the thickness of the water phase which is not covered with the monomolecular film to vary, and this variation causes the compression of the material to be inhomogeneous.
Furthermore, in an apparatus where the end position of the monomolecular film is close to the region S, there is a risk that part of the monomolecular film which has been formed in the region C might intrude into the region S. The LB film-forming material cannot be compressed uniformly in the region S. Furthermore, when the apparatus is in use, the material spread on the region S might not be transferred smoothly to the region C and material might be left in the region S which would bring about a local increase in the surface pressure. A homogeneous monomolecular film is difficult to obtain by such an apparatus.
As described above, the fluctuation of the surface pressure causes the water level to fluctuate, and also the fluctuation of the water level causes the surface pressure to fluctuate so the two control loops affect each other and the water level and the surface pressure cannot be controlled independently. This mutual influence is shown in FIG. 3 by a dotted line. Therefore, even when the parameters of the two loops are optimised, the fluctuations of the water level and the surface pressure are difficult to completely eliminate.
For example, where the surface pressure on the monomolecular film in the region D is set in the range from 20 to 40 mN/m and then controlled to maintain the set surface pressure, it is difficult to suppress the fluctuations to within 10% of the set value. In addition, if the surface pressure is set lower than 20 mN/m, the fluctuations get worse. Furthermore, the fluctuations tend to get worse with time. If a long time is required to produce an LB film, e.g. where a large substrate is used, a thick LB film is produced, in mass-production of an LB film, or when immersing and raising a substrate for depositing an LB film slowly etc. this tendency might cause some problems about the quality of the LB film.
One way for restricting the fluctuation of the surface pressure is increasing the slope of the water phase in the region C. However, in this case, the flow may become turbulent and the material may not be compressed homogeneously. The slope in the region C must be small, for example, only a few degrees in order that the flow is laminar. Another way for restricting the fluctuations of the surface pressure is keeping the end position of the monomolecular film sufficiently far away from the region S in order that the fluctuation of the area not covered by the monomolecular film does not influence the water level. However, this approach increases the size of the apparatus, and also prolongs the time needed to increase the surface pressure of the monomolecular film after supply of the film-forming material since the interval between spreading of the material on the liquid surface and the material reaching to the end of the monomolecular film which has been formed on the liquid surface is increased. In consequence the speed of response in the second control loop is reduced.